One common form of label applicator applies labels from a label strip to articles as the articles pass through a labeling station. The label strip includes a web or backing strip, and the labels which are spaced apart by a gap are removably adhesively secured to the backing strip. The label applicator removes the labels from the backing strip and transfers them to the articles at the labeling station.
More particularly, the label strip is indexed or moved intermittently during operation of the label applicator with each indexing movement of the label strip resulting in the removal of a label from the web and the transfer of that label, or a previously removed label, to the article at the labeling station. The stop signal for the label strip is typically, optically derived by directing light perpendicular to the plane of the label strip at a sensing station toward a photocell or other light receiver. In many cases, the labels transmit a relatively low percent of light, and the web transmits a higher percentage of light. Accordingly, there is a difference in intensity of the light received by the photocell, and this can be used to indicate the presence of each of the gaps at the sensing station.
Although such a label-sensing system works well for labels having relatively low light transmission characteristics, it cannot sense transparent labels or opaque labels on an opaque web. Various attempts have been made to sense labels in these situations, but each of them has drawbacks. For example, the web may be provided with an opaque spot or a hole which can be detected. However, these added operations increase the cost of the label strip as does the addition of a special ultraviolet opaque coating on the label adhesive. A rotating star wheel can be used to detect the thickness difference between the label-web combination and the web alone; however, this mechanism is speed-limited and cannot be used when rapid labeling is required.